With all lights/accessories OFF disconnect the field wires from the alt & at a fast idle jump 12V to either alt male field terminal & jump the other male field terminal to ground for 3 seconds max (just long enough to take a voltage reading). That'll tell you if the alt is OK. If good hook everything back up & see if the blue wire field circuit is hot at the alt and the reg. I'm assuming the brown wire you mentioned is a switched 12V? If so that wouldn't be an issue especially since it took awhile to act up. I run the blue field wire switched 12V to my Eddy elec choke also